Helena Parkington; Week 8 MED1011; Physiology
Membrane potential is measured using electrodes with a tiny diameter, cells are 6-75um. A patch clamp drags part of the membrane into the electrode can control the protein. Ion channels can be open and conducting, inactivated (non-conduction and unavailable) or closed (available). Closed to open is by activation stimulus, open to inactivated by sustained activation stinulus, inactivated to closed by removal of stimulus. Activation has change in membrane potential (depolarisation), release of neurotransmitter/hormone, rise in calcium within the cell, changes in cell nucleotides.
There are 10^6 channels and 10^2 pumps so deactivation is necessary to maintain membrane potential. Ion channels that are voltage gated are usually closed but available.
The action potential: Na enters through a few Na channels at first, initial slow depolarisation, induces opening of additional Na which reaches a threshold, rapid depolarisation overshoots 0mV, very fast and very brief, all or nothing. Goes to about 64mV/80mV. When inward Na current exceeds resting outward K, snowball effect really kicks in. There is a return to RMP with rapid inactivation of Na channels. Failure of inactivation can cause epilepsy.
Large depolarisation causes inactivation of voltage gated Na channels and activation of voltage gated K channels. Outward current is from K. There is induced after-hyperpolarisation that suppresses exciteability and speeds recovery of Na channels. Na/K ATPase is slow but mops up the problems with the membrane potential.
Na channels are found in nodes of Ranvier and Ca channels are found in neurotransmitters.